This invention is in the field of fluoropolymers having functional side groups, and is concerned with articles made from such fluoropolymers.
Thermoplastic fluoropolymers are well known for outstanding combinations of properties including chemical resistance, unique surface characteristics, high service temperatures, and good dielectric characteristics. As a result, fluoropolymer resins are used in a wide variety of applications including wire insulation, cable jacket, hose, tubing, film, linings for chemical process equipment, articles for fluid handling in laboratory and manufacturing situations, and the like. The service temperature in some of these applications can be high. As is common for thermoplastics, some properties of fluoropolymers change as temperature increases. Modulus and tensile strength, for example, typically decrease with increasing temperature, and fabricated articles such as tubing and wire insulation typically have lower cut-through resistance at high temperature.
Fluoropolymer articles having improved properties, particularly at high temperature, are desired.
The use of functional groups in fluoropolymers has been aimed at modifying the surface characteristics of fluoropolymers. For example, Kerbow in U.S. Pat. No. 5,576,106 discloses a grafted fluoropolymer powder that is effective in such uses as an adhesive to join dissimilar materials, Iura et al. in European Patent Application Publication EP 0 761 757 disclose a fluorine-containing polymer alloy of a grafted fluorine-containing polymer and a polymer containing no fluorine, and Shimizu et al. in European Patent Application Publication 0 626 424 disclose a blend of a thermoplastic resin and a fluoropolymer having a functional group that improves interfacial affinity with the thermoplastic resin.
This invention provides articles made from melt-fabricable fluoropolymer resin and having improved mechanical properties. Preferred such articles include tubing and wire insulation, which exhibit improved cut-through resistance.
The fluoropolymer resin from which such articles are made has pendant functional groups. Such groups can be produced by grafting a polar-functional compound to the fluoropolymer.
It has been discovered that addition of functional groups to fluoropolymers can result in surprising improvement of bulk properties. Improved properties are evident at room temperature, but are accentuated at elevated temperatures.
The articles of this invention can be fabricated by conventional melt processing techniques. For example, articles can be formed by melt extrusion, using such techniques as pressure extrusion or extrusion accompanied by melt draw, or by injection molding. The processes of fabricating the articles of the present invention are new in that the melt-fabricable fluoropolymer resin used has pendant functional groups.
Generally, when articles of this invention are fabricated by melt extrusion, a cross-section of such articles normal to the extrusion direction is characterized by a perimeter that is a closed figure. Such figures include, for example, regular geometric shapes such as circles, ellipses, rectangles and the like, and also irregular shapes including shapes that are either minor or major departures from regular shapes. Preferably, extruded articles of the invention have general axial symmetry though not necessarily exact axial symmetry. For example, convoluted tubing having helical convolutions does not have exact axial symmetry, but is considered for purposes of the present invention to have general axial symmetry (or near axial symmetry). Preferred extruded articles include wire insulation, cable jacket, tubing including convoluted tubing, hose, and the like.
Articles of the present invention exhibit improved mechanical properties. Such improvements include increased stiffness and increased toughness, the latter indicated, e.g., by increased force required to cut through a thin section. Such improvement can be evident at room temperature, but may be more pronounced at elevated temperature. For wire insulation of the present invention, substantial increases in cut-through resistance have been observed at 150xc2x0 C. See Example 1, which discloses an increase of about 100%. Such large increases are not required for articles of the invention. Cut-through resistance of wire insulation or tubing at 150xc2x0 C., for example, is generally at least 25% greater than for such articles made from comparable fluoropolymer resin but not having pendant functional groups, and is preferably at least 50% greater. Stiffness (flex modulus) is also increased. For example, flex modulus at room temperature is generally increased by at least 10% relative to that of the same article made from similar fluoropolymer lacking pendant functional groups.
The fluoropolymer having pendant functional groups exhibits a reduction in melt flow rate (increase in melt viscosity) after being exposed to elevated temperature such as may be encountered during melt fabrication of the article. This reduction in melt flow indicates that cross-linking occurs, even in the absence of cross-linking promoter or catalytic agent.
Articles of the present invention are fabricated essentially from fluoropolymer resin having pendant functional groups. The articles consist essentially of such resin in the sense that other substance or material which may react with said pendant functional groups is not present. Thus, for example, no cross-linking promoter or co-agent or such material is present. Likewise, no fluorine-free polymer is present as a component of a blend or alloy, or as a reinforcing component. However, articles of the invention can contain inert additives present in minor amount, such as pigments, or present merely as filler, such as carbon. Additives such as thermal or oxidative stabilizers that are normally present in fluoropolymer resins can be present in usual amounts in articles of the invention.
As used herein, xe2x80x9cfunctionalized fluoropolymerxe2x80x9d means fluoropolymer having functional side groups or functional groups attached to side groups, i.e., pendant functional groups. Usually, but not necessarily, such functional units are at the ends of the pendant side groups. Fluoropolymer that does not have such pendant functional groups is sometimes described herein as xe2x80x9cnon-functional fluoropolymerxe2x80x9d. Thus, non-functional fluoropolymer and functionalized fluoropolymer differ at least by the presence in the latter of pendant functional groups. Non-functional fluoropolymer can be a precursor to functionalized fluoropolymer, in which instance the process of functionalizing involves addition of functional groups to the non-functional polymer. However, xe2x80x9cfunctionalizingxe2x80x9d is also used in a broader sense herein to include preparation of functionalized fluoropolymer which would be non-functional if pendant functional groups were not present, even though non-functional fluoropolymer may not be the precursor.
Functional groups, in the context of the present invention, are groups capable of improving the mechanical properties of fluoropolymer articles when such functional groups are present in a fluoropolymer composition. Improvement of mechanical properties is indicated, for example, by an increase in cut-through resistance (e.g., as measured for wire insulation or for tubing) or by an increase in stiffness (flex modulus), as discussed above. Functional groups that can improve the properties of fluoropolymers when present include ester, alcohol, acid (including carbon-, sulfur-, and phosphorus-based acid) and salt and halide thereof. Other functionalities include anhydride and epoxide. Preferred functional groups include xe2x80x94CH2xe2x80x94OH and anhydride, especially maleic anhydride. As one skilled in the art will recognize, more than one type of functional group can be present. Normally, however, a single type of functional group is used.
Such functional groups can be introduced, for example, by incorporating into the fluoropolymer, during polymerization, monomer units having such functional groups, i.e., functional monomers, or by having an ethylenically unsaturated compound grafted thereto which imparts polar functionality to the fluoropolymer, the polar functionality being present as part of the ethylenically unsaturated compound. Such grafted fluoropolymer includes the grafted fluoropolymer powder described in U.S. Pat. No. 5,576,106 and the grafted fluoropolymer described in EP 0 650 987. Other known methods of grafting can be used. Preferred polar-grafted fluoropolymers include the surface-grafted powder of the ""106 patent. Examples of polar functionality provided by grafting include acids, including carboxylic, sulfonic and phosphonic acids, and esters and salts thereof, and epoxides. Glycidyl methacrylate is an example of a grafting compound that provides epoxide functionality. Among compounds for grafting onto and thereby becoming part of the polar-grafted fluoropolymer, maleic acid and maleic anhydride are preferred. Maleic anhydride can be halogen-substituted, e.g., dichloromaleic anhydride and difluoromaleic anhydride.
The concentration of functional groups in the fluoropolymer resin component, i.e., in the functionalized fluoropolymer or in blends of functionalized fluoropolymer plus non-functional fluoropolymer, if non-functional fluoropolymer is present, of the melt-fabricable fluoropolymer composition of this invention is effective to improve mechanical properties of the article fabricated by melt extrusion or injection molding. For example, the cut-through resistance at 150xc2x0 C. of wire insulation, tubing, or any other article that can be subjected to the cut-through test is improved by at least 25% relative to the same article made from comparable fluoropolymer but having no pendant functional groups, and is preferably improved by at least 50%. Alternatively, the flex modulus at room temperature is increased by at least 10% over the same article made from similar fluoropolymer having no pendant functional groups. As will be recognized by one skilled in the art, the concentration of functional groups that is effective to achieve improvement may vary at least with the type of functional group. The concentration of functional groups present can be expressed relative to the number of main chain carbon atoms in the fluoropolymer resin. Generally, the concentration of functional groups present is at least about 25/106 main chain C atoms, based on total fluoropolymer in the composition. The concentration of functional groups is usually in the range of 25-2500 per 106 main chain C atoms, preferably in the range of 50-2000 per 106 main chain C atoms, based on total fluoropolymer present.
The desired concentration of functional groups in the functionalized fluoropolymer resin can be achieved with a single fluoropolymer having functional groups, or a mixture of such fluoropolymers. The desired concentration of functional groups can also be achieved by blending functionalized fluoropolymer (or mixture of) having a higher concentration of functional groups with non-functional fluoropolymer (or mixture of), i.e., fluoropolymer having essentially no functional groups. In this embodiment, functionalized fluoropolymer acts as a functional group concentrate that can be let down (diluted) with non-functional fluoropolymer. This approach has the advantage of permitting one to achieve a variety of functional group concentrations with a single functionalized fluoropolymer by varying the blending ratio with non-functional fluoropolymer, and is a preferred embodiment of the invention. Preferably, in a functionalized fluoropolymer that is a blend, the functionalized fluoropolymer component is in minor (lesser) amount relative to non-functional fluoropolymer component.
Thus, in one embodiment of the present invention, the cross-linkable fluoropolymer composition contains minor amounts of functionalized fluoropolymer and a major amount of non-functional fluoropolymer. By xe2x80x9cmajor amountxe2x80x9d is meant at least 50 wt %, preferably at least 70 wt %, of non-functional fluoropolymer based on combined weight of non-functional fluoropolymer and functional fluoropolymer. In this embodiment of the invention, then, the concentration of functional groups in the functionalized fluoropolymer will be high enough so that the average concentration of functional groups in the functional fluoropolymer plus the non-functional fluoropolymer will be at least about 25/106 main chain C atoms, usually in the range of 25-2500 per 106 main chain C atoms, and preferably in the range of 50-2000 per 106 main chain C atoms.
A wide variety of fluoropolymers can be used. The fluoropolymer is made from at least one fluorine-containing monomer, but may incorporate monomer, which contains no fluorine or other halogen. Fluorinated monomers include those which are fluoroolefins containing 2 to 8 carbon atoms and fluorinated vinyl ether (FVE) of the formula CY2xe2x95x90CYOR or CY2xe2x95x90CYORxe2x80x2OR wherein Y is H or F and xe2x80x94Rxe2x80x94 and xe2x80x94Rxe2x80x2xe2x80x94 are independently completely fluorinated or partially fluorinated linear or branched alkyl and alkylene groups containing 1 to 8 carbon atoms. Preferred R groups contain 1 to 4 carbon atoms and are preferably perfluorinated. Preferred Rxe2x80x2 groups contain 2 to 4 carbon atoms and are preferably perfluorinated. Hydrocarbon monomers that can be used include ethylene, propylene, n-butylene, and iso-butylene. When the fluoropolymer is to be functionalized by grafting, preferably at least one monomer contains hydrogen, and in that regard the hydrogen/fluorine atomic ratio in the polymer is preferably at least 0.1/1. The fluoropolymer, however, preferably contains at least 35 wt % fluorine. Fluoropolymer resins that can be used include copolymers of TFE with one or more copolymerizable monomers chosen from perfluoroolefins having 3-8 carbon atoms and perfluoro(alkyl vinyl ethers) (PAVE) in which the linear or branched alkyl group contains 1-5 carbon atoms. Preferred perfluoropolymers include copolymers of TFE with at least one of hexafluoropropylene (HFP) and PAVE. Preferred comonomers include PAVE in which the alkyl group contains 1-3 carbon atoms, especially 2-3 carbon atoms, i.e. perfluoro(ethyl vinyl ether) (PEVE) and perfluoro(propyl vinyl ether) (PPVE). Preferred fluoropolymers also include the copolymers of ethylene with perhalogenated monomers such as TFE or chlorotrifluoroethylene (CTFE), such copolymers being often referred to as ETFE and ECTFE, respectively. In the case of ETFE, minor amounts of additional monomer are commonly used to improve properties such as reduced high temperature brittleness. PPVE, PEVE, perfluorobutyl ethylene (PFBE), and hexafluoroisobutylene (HFIB) are preferred additional comonomers. ECTFE may also have additional modifying comonomer. Other fluoropolymers that can be used include vinylidene fluoride (VF2) polymers including homopolymers and copolymers with other perfluoroolefins, particularly HFP and optionally TFE. TFE/HFP copolymer which contains a small amount of VF2, which copolymer is often referred to as THV, can also be used. Examples of perfluorinated copolymers include TFE with HFP and/or PPVE or PEVE. Representative fluoropolymers are described, for example, in ASTM Standard Specifications D-2116, D-3159, and D-3307. Such fluoropolymers are usually partially-crystalline as indicated by a non-zero heat of fusion associated with a melting, endotherm as measured by DSC on first melting. Alternatively or additionally, preferred fluoropolymers are non-elastomeric, as opposed to elastomeric.
Functionalized fluoropolymers include fluoropolymers such as those described in the foregoing paragraph and additionally containing copolymerized units derived from functional monomers. If the concentration of functional monomer is high enough in a TFE copolymer, however, no other comonomer may be needed. Usually, but not necessarily, the functional groups introduced by such monomers are at the ends of pendant side groups. Examples of functional monomers that introduce pendant side groups having desired functionality include the same ethylenically unsaturated compounds recited above as grafting compounds. Such functional monomers can be incorporated into fluoropolymers, for example, by polymerization in a medium of CO2 as illustrated by example below. Functional monomers that introduce pendant side groups having desired functionality can also have the general formula CY2xe2x95x90CYxe2x80x94Z wherein Y is H or F and Z contains a functional group. Preferably, Y is F and xe2x80x94Z is xe2x80x94Rfxe2x80x94X, wherein Rf is a fluorinated diradical and X is a functional group that may contain CH2 groups. The functional groups include ester, alcohol, acid (including carbon-, sulfur-, and phosphorus-based acid) and salt and halide thereof, anhydride and epoxide. Preferred functional groups include xe2x80x94CH2xe2x80x94OH and anhydride, especially maleic anhydride. As one skilled in the art will recognize, more than one type of functional group can be present. Normally, however, a single type of functional group is used. Preferably, Rfxe2x80x94X is linear or branched perfluoroalkoxy having 2-20 carbon atoms, so that the functional comonomer is a fluorinated vinyl ether. Examples of such fluorovinylethers include CF2xe2x95x90CF[OCF2CF(CF3)]mxe2x80x94Oxe2x80x94(CF2)nCH2OH as disclosed in U.S. Pat. No. 4,982,009 and the alcoholic ester CF2xe2x95x90CF[OCF2CF(CF3)]mxe2x80x94Oxe2x80x94(CF2)nxe2x80x94(CH2)pxe2x80x94Oxe2x80x94COR as disclosed in U.S. Pat. No.5,310,838. Additional fluorovinylethers include CF2xe2x95x90CF[OCF2CF(CF3)]mO(CF2)nCOOH and its carboxylic ester CF2xe2x95x90CF[OCF2CF(CF3)]mO(CF2)nCOOR disclosed in U.S. Pat. No. 4,138,426. In these formulae, mxe2x95x900-3, nxe2x95x901-4, pxe2x95x901-2 and R is methyl or ethyl. Preferred such fluorovinylethers include CF2xe2x95x90CF[OCF2CF(CF3)]O(CF2)2xe2x80x94CH2xe2x80x94OH. These fluorovinylethers are useful because of their ability to incorporate into the polymer and their ability to incorporate functionality into the resultant copolymer. Preferred comonomers that introduce pendant functional groups include maleic anhydride, dichloromaleic anhydride, difluoromaleic anhydride, and maleic acid.
When functionalized fluoropolymer is achieved by copolymerization, the amount of functional monomer in the functionalized fluoropolymer of this invention is small to achieve the desired concentration of functional groups, even when functionalized fluoropolymer is a blend comprising non-functional fluoropolymer. Generally, the amount of functional monomer is no more than 10 wt %, preferably no more than 5 wt %, based on total weight of functionalized fluoropolymer, i.e., the fluoropolymer component containing the functional monomer. In certain instances, higher, concentrations of functional monomer exceeding 10 wt % may be desired, for example, when it is not desired to use a non-functional monomer in the functionalized melt-fabricable fluoropolymer. While the functionalized fluoropolymer can be uniform, it is not necessary to have a uniform concentration of functional monomer throughout the functionalized fluoropolymer.
When pendant functional groups are introduced into the melt-fabricable fluoropolymer by, e.g., maleic anhydride or maleic acid, including the halogen-substituted counterparts, either by grafting or by copolymerizing, the amount of grafting compound grafted to the fluoropolymer or the amount of functional comonomer incorporated into the fluoropolymer will generally be in the range of 0.01-1.0 wt %, preferably 0.02-0.5 wt %, based on total fluoropolymer present in the composition. If the composition contains both non-functional fluoropolymer and functionalized fluoropolymer, the functionalized fluoropolymer will have larger amounts of grafted compound or copolymerized comonomer units depending on the proportion of functionalized fluoropolymer in the composition. Generally, the amount of maleic anhydride/acid is in the range of 0.05 wt % to 5 wt % based on the weight of functionalized fluoropolymer in such a fluoropolymer blend. Preferably, the amount of maleic anhydride/acid in the functionalized fluoropolymer component of a blend is 0.1-3 wt %, more preferably0.1-1 wt %.